Coexistence of IgLON5-IgG and SOX1-IgG in a Patient with Progressive Brainstem Dysfunction.

Purpose: The coexistence of IgLON5-IgG and SOX1-IgG is rare. Previous reports have shown that patients with IgLON5-IgG spectrum disease present with sleep disorders, bulbar involvement, and autonomic abnormality, while SOX1-IgG positive patients present with peripheral nervous system symptoms such as the Lambert-Eaton Myasthenic Syndrome (LEMS).

Case report: We report a patient who presented with progressive ophthalmoplegia, ptosis, oropharyngeal dysphagia, gait instability, and sleep disorders. The paraneoplastic antibody screening tested doublepositive for IgLON5-IgG and SOX1-IgG. However, there was no clinical sign of LEMS in this patient. After extensive cancer screening, only lung nodules with hilar adenopathy were noted.

Conclusion: The coexistence of IgLON5-IgG with onconeuronal SOX1-IgG would suggest an underlying immune-mediated paraneoplastic process rather than secondary autoimmunity because of neurodegeneration. This is the first IgLON5-IgG case reported in Thailand, with a case of doublepositive IgLON5-IgG and SOX1-IgG as well. Keyword: IgLON5-IgG, SOX1-IgG, Paraneoplastic process, case report.